In application, the streamers are usually towed behind a vessel, either a ship or submarine. In active sensing, an acoustic noise source insonifies an area and the streamer detects reflections from any targets in the area. In passive sensing the streamer merely "listens" for any noise generated by targets. This invention describes techniques that may be applied to streamers used in either application.
An axial section of a prior art streamer structure is given in FIG. 1 and will be familiar to those skilled in the art. Connector 1 mechanically, electrically and optically connects each streamer section to the one before it and thence ultimately to the towing vessel. The signal from hydrophone 2 (only one hydrophone is shown) is taken to connector 1 via wires 3. Strength members 4 run from connector to connector and provide the axial mechanical strength. A core material, either solid or liquid, fills the space 5 to provide buoyancy (and in some cases desirable acoustic properties). The streamer section is enclosed with a continuous skin 6, which protects the in-streamer components from water ingress.
A critical performance criterion of streamers is the self noise of the streamer. That is, the unwanted acoustic signal that is detected by the hydrophones when the streamer is towed through the water. The self noise has two major components, that caused by the turbulence generated by towing the streamer through the water (flow noise) and that induced by the vibration of the streamer structure. A large part of the vibration coupled into the streamer originates from the tow vessel and is transmitted to the hydrophone section via the towing cable and strength members. Other sources of vibration may also be present, but are usually less important.
In liquid filled streamers the vibration noise is dominated by a single propagating wave and can be reduced by designing a group of hydrophones which are spaced in the direction of propagation, the spacing being a function of the wavelength of the propagating wave. The signals from the group of hydrophones are then summed into a single channel and the wave effects thus cancel each other. Although this technique is successful at noise reduction over a limited frequency range at the design temperature, the performance cannot be sustained for the temperature range over which the seismic streamer has to operate due to changes in the characteristics of the propagating wave. Typically the jacket material has to be changed to sustain the performance.
Hitherto, liquid filled streamers have been used since the liquid fill has been shown to have favourable properties with regard to self noise, but liquid filled streamers are fragile and can be inconvenient in operation. For example, damage to one section of the streamer during steady state towing can lead to a "knock-on effect" whereby all the sections fail progressively as the streamer sinks. Damage to a section of solid-filled streamer will not necessarily lead to failure of that particular section or any other section and, accordingly, solid-filled streamers are generally preferred.
Liquid filled streamers are also susceptible to damage during recovery and deployment which is normally achieved by winching. In order to minimise this damage streamers are recovered at low boat speeds and winching speeds. The low recovery speeds and susceptibility to damage during winching can lead to loss of valuable survey time with a consequent loss of revenue. This becomes more evident when data is being collected using multiple streamers. The use of solid streamers allows recovery at the normal operating speeds with loss of data only from the streamer being recovered. The recovery time is also reduced by winching at line speeds higher than that which can be achieved with liquid filled streamers.
Efforts at replacing liquid filled with more robust solid filled streamers have been frustrated by difficulties in reducing the self noise of solid streamers to acceptable levels.
In conventional streamer structures such as that shown in FIG. 1, vibration at the head of the streamer couples via the strength members 4 and connector plates 1 into the core material 5 of the streamer which is liquid. This then generates a slow speed bulge or breathing wave of high amplitude, compared with the acoustic signals normally received by the hydrophones, thus increasing the overall self noise of the streamer. If the core material 5 is solid the performance is usually worse, unless the carrier disclosed herein is incorporated.